1. Field of the Invention
This invention relates to a photocoupler.
2. Background Art
A photocoupler can transmit signals between units with their power supply systems insulated from each other, and is required to transmit and receive a large amount of control signals and digital signals, such as on a bus connecting between the units. To construct such a signal transmission system, a compact, lightweight photocoupler having a small number of terminals is needed.
In a photocoupler, an input electrical signal is converted to an optical signal by a semiconductor light emitting device. The photocoupler often includes a drive circuit for driving the semiconductor light emitting device. In general, a semiconductor light emitting device has a wide range of characteristic distribution, and temperature variation also occurs in the characteristics. If the characteristics of the semiconductor light emitting device packaged in a photocoupler can be inspected without being affected by the drive circuit, the operating margin can be accurately defined, and the reliability is easily ensured.
Japanese Patent No. 3367718 discloses a technique for inspecting a semiconductor light emitting device without increasing the number of terminals. In this technique, the inspection mode is set to a region in which the power supply voltage is lower than the guaranteed operating range, and the signal input is converted to a current input to the semiconductor light emitting device for inspection. However, this technique has some problems. For example, the inspection mode is entered during the rising and falling of the power supply voltage. Furthermore, the operating range of the inspection mode is narrow at a low power supply voltage.